That command will split bigfile.iso into files that are 1024 MB in size (1GB) and name the various parts small_file_aa, small_file_ab, etc. You can specify b for bytes, k for Kilobytes and m for Megabytes to specify sizes.

To join the files back together on Linux:

cat small_file_* > joined_file.iso

Similarly to join the split files on a Windows machine, use the copy command: